A high-intensity discharge lamp (HID lamp), which is one type of high-pressure discharge lamp, is widely used in various fields in view of high luminance intensity and selectability of high-efficiency type. Particular, in late years, a metal halide lamp is used as spotlights and downlights in indoor stores by taking advantage of its high color rendering performance. For this reason, an appearance design of a lamp fitting becomes important, and there is the need for more compact lamp fittings. Consequently, instead of a lighting apparatus with an integrated structure of a lamp fitting adapted to mount a lamp and a ballast serving as a lighting device, a lighting apparatus comprising a light fitting and a ballast disposed apart from one another and electrically connected to one another through a wiring, such as a cable, is becoming popular.
Particularly, in a lighting apparatus designed to output a high-voltage pulse from a ballast so as to start up a lamp, the high-voltage pulses continuously applied to a cable are liable to deteriorate the wiring. Thus, a wiring capable of withstanding an integral stress of the applied high-voltage pulses has to be used. This requirement is disadvantageous, for example, in terms of cost. The following Parent Publication 1 discloses a lighting device (hereinafter referred to as “conventional device 1”) intended to solve such a problem.
The conventional device 1 comprises a first timer for counting a time (typically 10 seconds) required for initial start-up of a high-pressure discharge lamp (required for allowing a high-pressure discharge lamp to initially start up), a second timer for intermittently activating the first timer in constant time cycles (typically 2 minutes), and a third timer for activating each of the first and second timers for at least a time equal to or greater than a time (typically 20 minutes) required for restart of the high-pressure discharge lamp (required for allowing the high-pressure discharge lamp to have a restartable condition). The conventional device 1 is designed to activate an igniter only within the counting time of the first timer and inhibit the igniter from operating after elapse of the counting time of the third timer. That is, the conventional device 1 is designed to allow an operation of the igniter for a time sufficient for the initial start-up of the high-pressure discharge lamp to be repeatedly performed within a time sufficient for the restart of the high-pressure discharge lamp. This makes it possible to minimize the occurrence of electric noise due to the high-voltage pulses in a non-lighted state of the lamp, and the risk of deterioration of the wiring.
The conventional device 1 is a ballast using a magnetic circuit (so-called “copper-iron ballast”). Recent years, in connection with the need for reduction in weight and size and enhancement of functionality of a ballast, the mainstream of lighting devices is being shifted to an electronic ballast using a number of electronic components.
FIG. 25 is a circuit block diagram showing one example of a conventional electronic ballast (hereinafter referred to as “conventional device 2”). The conventional device 2 comprises a rectification circuit 1 for full-wave rectifying a voltage from an AC power supply AC which is a commercial power supply, a step-up chopper circuit 2 for converting a pulsating voltage rectified through the rectification circuit 1 into a desired DC voltage, a step-down chopper circuit 3 for stepping down an output DC voltage from the step-up chopper circuit 2, a polarity reversing circuit 5 for alternating an output DC voltage from the step-down chopper circuit 3 at a low frequency of several ten to several hundred Hz to apply a rectangular-wave voltage to a high-pressure discharge lamp 4 (hereinafter referred to as “discharge lamp 4”), and an igniter circuit 31 for applying start-up high-voltage pulses to the discharge lamp 4.
The step-up chopper circuit 2 has a commonly-known configuration which includes a chopper choke 8, a rectifying element 7, a switching element 6 and a smoothing capacitor 9. A first control circuit 10 is operable to PWM-control the switching element 6 so as to obtain a DC output voltage Vdc stepped up to a desired level, between both ends of the smoothing capacitor 9. The step-down chopper circuit 3 has a commonly-known configuration which includes a switching element 11, a rectifying element 12, a chopper choke 13 and a smoothing capacitor 14. A second control circuit 15 is operable to PWM-control the switching element 11 so as to obtain a DC output voltage stepped down to a desired level, between both ends of the smoothing capacitor 14. The step-up chopper circuit 2 and the step-down chopper circuit 3 each having the above configuration are commonly known, and the detailed description of their operations will be omitted.
The igniter circuit 31 includes a pulse transformer 20 having a secondary winding inserted between the polarity reversing circuit 5 and the discharge lamp 4, and a pulse generator 21 for applying a pulse voltage to a primary winding of the pulse transformer 20. The igniter circuit 31 is operable to superimpose the high-voltage pulses on the rectangular-wave voltage having a polarity reversed through the polarity reversing circuit 5 so as to start up the discharge lamp 4.
The inductor 8 of the step-up chopper circuit 2 is provided with a secondary winding. An AC voltage induced in this secondary winding is rectified, limited and smoothed, respectively, through a diode 18, a resistor 19 and a capacitor 16 to obtain an operating power for the first and second control circuits 10, 15. In this case, a voltage between both ends of the capacitor 16 can be increased up to a value equal to or greater an operating voltage of the first and second control circuits 10, 15 only if the step-up chopper circuit 2 operates to allow a current to flow through the inductor 6 at a given value or more. It is also required to stabilize an output of the capacitor 16 using a three-terminal regulator or the like.
Parent Publication 1: Japanese Patent No. 63307695